1. Field of the Invention
The present invention relates to time measurement devices and, more particularly, to time-to-digital converters.
2. Description of the Relevant Art
Time-to-digital converters (TDC) are used in various applications to measure the time at which an event occurs. The occurrence of an event is detected via a trigger signal input. The TDC detects when the trigger signal, or trigger event, occurs relative to a reference clock signal. Although the reference clock signal may be very precise, it is often necessary to resolve the occurrence of the trigger signal with more precision than the period of the reference clock. Therefore, it is necessary to determine at what point within a clock period an event occurs.
Many techniques exist for measuring the time delay from a trigger event to a reference clock signal. For example, dual slope devices use "expanded time" to measure time delays. In dual slope devices, the arrival of the trigger event initiates the charging of an integration capacitor. The charging stops on the subsequent edge of a reference clock signal. The voltage of the capacitor is thus a measure of the time delay between the trigger event and the reference clock. The capacitor is then discharged at a rate slower than the charge rate, and the discharge time is measured. The discharge time is measured by counting the number of reference clock pulses that occur before the capacitor is fully discharged. The slower discharge time of the capacitor provides a convenient way to accurately measure the charge time, or time delay, using a slower clock rate. Dual slope devices are problematic because they require relatively long times to resolve the time delay. Further, inaccuracies in the charge and discharge currents and capacitive values may effect the accuracy of the time measurement.
FIG. 1 illustrates another embodiment of a time-to-digital converter. This device is referred to as a quadrature TDC. The quadrature TDC includes a quadrature hybrid, a synchronizer, two track-and-hold circuits, two analog-to-digital converters (ADC), and an encoding circuit. The quadrature hybrid outputs two continuous signals, called quadrature signals, that are 90 degrees out-of-phase from each other. The quadrature signals are synchronized with the reference clock. The quadrature TDC determines the time delay relative to the reference clock by measuring the amplitude of the outputs of the quadrature hybrid. The amplitude of the quadrature signals indicate the time at which a trigger event occurred. The quadrature signals must be continuous signals for the quadrature TDC to operate correctly. For example, the quadrature signals may be sinusoids or trapezoidal. Because the quadrature signals are continuous, it is necessary to detect the amplitude of multiple signals. Continuous signals reach the same amplitude at least two times each cycle. Therefore, the quadrature hybrid outputs two signals that are 90 degrees out-of-phase. By measuring the amplitude of both quadrature signals, it is possible to accurately determine at which point the trigger event occurred. Unfortunately, the quadrature TDC requires the generation of multiple clock signals and requires multiple track-and-hold circuits and multiple analog-to-digital converters. Quadrature TDC's are discussed in more detail in U.S. Pat. No. 5,191,336 to Paul Stephenson entitled "Digital Time Interpolation System."
What is desired is a TDC that accurately measures beyond the resolution of a reference clock without the need for multiple reference signals. Additionally, a TDC that compensates for inaccuracies in charge and discharge currents is desired.